1. Field of the Invention
The present invention relates to a scheme for paging in a mobile communication system having a plurality of base stations provided in respective cells, where each base station is transmitting a paging channel on which a paging signal for each mobile station is transmitted at intermittent timings such that each mobile station judges a presence or absence of a paging signal for the own station by receiving the paging channel at these intermittent timings.
2. Description of the Background Art
In the cellular mobile communication system, at a time of call termination for a mobile station located within a cell of a base station, the base station carries out the paging with respect to the mobile station by using a downlink radio channel called paging channel.
Usually, each frame of this paging channel (PCH) is divided along the time axis into a plurality of time-slots (TS) as shown in FIG. 1, where which time-slot of the paging channel frame is to be used for the paging with respect to which mobile station is determined in advance. Consequently, while waiting for a call to the own station, it suffices for the mobile station to receive only the specific time-slot of the paging channel frame intermittently and there is no need to constantly receive the paging channel.
By increasing a number of time-slots to be used, it is possible to reduce a rate (referred hereafter as an intermittent ratio) of a time period for which the mobile station actually receives the paging channel with respect to the entire time period of the paging channel frame, and consequently it is possible to reduce the power consumption at a time of call waiting by the mobile station considerably compared with a case of constantly receiving the paging channel.
In an example shown in FIG. 1, each frame of the paging channel is divided into right time-slots, and the mobile station receives only one time-slot (TS3 in an example of FIG. 1) among them at a time of call waiting. Consequently, in this example, the intermittent ratio is 1/8 so that the power consumption of the mobile station at a time of call waiting can be reduced to 1/8.
Now, in the conventional mobile communication system, the intermittent ratio has been reduced by increasing a number of time-slots. However, when a number of time-slots is increased, the paging interval also increases as shown in FIG. 2, and consequently, an average time required for the paging is also increased so that there has been a limit to an extent by which the intermittent ratio can be reduced by the increase of a number of time-slots. In an example shown in FIG. 2, a number of time-slots is increased to twelve from eight used in FIG. 1, so that the intermittent ratio is reduced from 1/8 to 1/12, but the average time required for the paging is also increased accordingly.
It is also possible to reduce the paging interval by reducing a data amount necessary for the paging and thereby reducing a unit time-slot duration, but there is also a limit to an extent by which the data amount necessary for the paging can be reduced, so that there is also a limit to an extent by which the intermittent ratio can be reduced in this way.